Field of the Invention
The present invention relates to a discharge device which discharge power stored in an electrical storage element, and in particular to a discharge device which, using a control signal, drives a switch element connected in parallel to the electrical storage element.
Description of the Related Art
An inverter, or the like, which controls a motor, by controlling a switching device, switches a current supply path through which to cause current to flow to each coil of the motor from a power supply, thus carrying out a drive control of the motor. Also, a transformer, or the like, by controlling the switching device, adjusts the amount of current supplied to a reactor (a coil) from the power supply, and transforms a voltage generated in the power supply into an optional voltage, thus carrying out an output of the voltage.
As a specific configuration of the switching device, there is one wherein a first switch element and a second switch element are connected in series, and the connection point of the first switch element and second switch element is connected to the motor or reactor as an output portion. Further, an electrical storage element (generally, a capacitor) which smooths a power fluctuation is provided in connection with the first switch element and second switch element connected in series. Further, when disconnecting wires from the motor and inverter in order to maintain, overhaul, and repair the inverter and motor after the operation of the inverter or the like is over, working efficiency is low when electric charge is stored in the electrical storage element of an inverter input stage, meaning that it is necessary to discharge energy (electric charge) with which the electrical storage element is charged.
In order to discharge the electric charge of the electrical storage element, attention is focused on an energization loss due to the internal resistance of a switch element, and it is proposed in Patent Document 1 that a first switch element and a second switch element are connected in series, and that the first switch element is connected to the positive terminal side of the electrical storage element, while the second switch element is connected to the negative terminal side of the electrical storage element, so that the first switch element and second switch element connected in series are connected in parallel to the electrical storage element, and the two electrodes of the electrical storage element are short-circuited by repeating an on-state of the switch elements at the same time for a very short time, thus discharging the electric charge of the electrical storage element by only switching operation of the switch elements included in advance.
However, when current flows through a switch element, a loss occurs due to the integration of a voltage applied to the switch element and a current flowing through the switch element, and electric charge stored in the electrical storage element can be discharged, but when an energization current of the switch element increases, the loss of the switch element increases, and the amount of heat generation also increases. Because of this, when the energization current of the switch element increases excessively when discharging, the loss and heat generation of the switch element increase, leading to a thermal destruction of the switch element at the worst.
Consequently, when discharging the electric charge of the electrical storage element, the discharge device has to suppress the energization current of the switch element and avoid the switch element resulting in a thermal destruction.
For example, in the discharge device described in Patent Document 1, when carrying out discharge, the energization current of the switch element is prevented from becoming an overcurrent by defining an on-time (a current rise time) of a drive signal of the switch element and turning off the switch element in a region in which the energization current of the switch element is lower than a limiting current Icmax. Further, turning on and off of the switch element is carried out a plurality of times until the electrical storage element is discharged, and the energization current of the switch element is suppressed, thus avoiding the switch element resulting in a thermal destruction.
Also, in an embodiment of the discharge device described in Patent Document 1, it is possible to suppress the energization current of the switch element by supplying a switch element drive voltage in a region (a current saturation region) in which the energization current of the switch element is lower than the limiting current Icmax (that is, by carrying out a reduction in the switch element drive voltage and supplying the reduced drive voltage), during a discharge control time, compared with during a normal time. Consequently, when discharging the electrical storage element, the energization current of the switch element is suppressed, thus avoiding the switch element resulting in a thermal destruction.
[Patent Document 1] JP-A-2009-232620
However, in the discharge device proposed in Patent Document 1, the on-time (current rise time) of the drive signal of the switch element is defined during the discharge control time, but a rise of current occurs for the on-time of the drive signal of the switch element. That is, when the on-voltage threshold value of the switch element or more is reached upon receiving an on-instruction by the drive signal, current starts to flow through the switch element, and the current increases throughout the on-time. Further, the switch element is turned off by the drive signal in a region in which the energization current of the switch element is lower than the limiting current Icmax.
In Patent Document 1, a voltage drive type switch element (for example, a MOSFET or an IGBT) is shown, and in this case, an on-time (a current rise time) tr can be expressed by Equation 1, wherein Rg is a switch element input resistance, Ciss is a switch element input capacitance, Vg is a switch element drive voltage, Vth is a switch element on-voltage threshold value, Ic is a switch element energization current, and gm is a switch element transmission coefficient, and by inputting an optional current value into Ic, it is possible to obtain the current rise time tr.
                              t          r                =                              -                          R                              g                ×                                              ×                      C            iss                    ×                      ln            ⁡                          (                              1                -                                                      1                                                                  V                        g                                            -                                              V                        th                                                                              ⁢                                                                                    I                        c                                                                    g                        m                                                                                                        )                                                          Equation        ⁢                                  ⁢        1            
The feature of the voltage drive type switch element is that the energization current of the switch element is saturated at a certain current value in a region in which the switch element drive voltage is low. That is, in the region in which the switch element drive voltage is low, an optional switch element drive voltage can be suppressed to an optional energization current of the switch element. Because of this, in the discharge device of Patent Document 1, the switch element drive voltage is controlled to the optional switch element drive voltage so as to be lower than the value of a switch element drive voltage applied in normal operation so as to obtain the optional energization current of the switch element.
Meanwhile, in general, the on-voltage threshold value and the switch element drive voltage vary due to difference among individual switch elements and to variability among parts of a switch element drive voltage supply circuit.
Because of this, in the discharge device of Patent Document 1, when the preset switch element on-voltage threshold value Vth is large, or when the switch element drive voltage Vg is small, the defined on-time tr increases, and the energization current of the switch element becomes larger than a supposed switch element energization current. As a result, when the on-voltage threshold value Vth and the switch element drive voltage Vg vary, the switch element energization current cannot be suppressed when discharging the electrical storage element, thus causing the problem that the switch element is heated to a temperature higher than expected.
Also, in the case of setting the switch element drive voltage Vg to be low due to the previously described variation, when the on-voltage threshold value Vth is high, and the switch element drive voltage Vg is much lower than the optionally set switch element drive voltage Vg, the on-voltage threshold value Vth>the switch element drive voltage Vg, and there is also a possibility that the switch element is not driven, thus disabling discharge.